function y = gampdf(x,a,b)
%GAMPDF Gamma probability density function.
%   a is shape and b is scale
%   Y = GAMPDF(x,a,b) returns the gamma probability density function 
%   with parameters a and b, at the values in X.
%   The mean is a*b and the variance is a*b*b
%
%   The size of Y is the common size of the input arguments. A scalar input  
%   functions as a constant matrix of the same size as the other inputs.    
%
%   Some references refer to the gamma distribution with a single
%   parameter. This corresponds to the default of B = 1.

%   References:
%      [1]  L. Devroye, "Non-Uniform Random Variate Generation", 
%      Springer-Verlag, 1986, pages 401-402.


if nargin < 3, 
    b = 1; 
end

if nargin < 2, 
    error('Requires at least two input arguments'); 
end

 [errorcode x a b] = distchck(3,x,a,b);

 if errorcode > 0
    error('Requires non-scalar arguments to match in size.');
end

% Initialize Y to zero.
y = zeros(size(x));

%   Return NaN if the arguments are outside their respective limits.
y(a <= 0 | b <= 0) = NaN;     

k=find(x > 0 & ~(a <= 0 | b <= 0));
if any(k)
    y(k) = (a(k) - 1) .* log(x(k)) - (x(k) ./ b(k)) - gammaln(a(k)) - a(k) .* log(b(k));
    y(k) = exp(y(k));
end
y(x == 0 & a < 1) = Inf;
k2 = find(x == 0 & a == 1);
if any(k2)
  y(k2) = (1./b(k2));
end

